24 V DC power supply sizing calculator
List the loads on a panel's 24 V rail, add headroom, and see the supply rating to look for. Fill in figures from each datasheet. The example values shown are placeholders.
Total load
2.62A
With 25% headroom
3.28A
Look for a supply of at least
5A, 24 V DC
The load would be about 52 per cent of that supply's rating.
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How it works
Add the maximum current of every load: PLC CPU and I/O, relays and contactor coils, sensors, HMI and network gear. Use the datasheet maximum, not a typical figure.
Add headroom of 20 to 30 per cent for future additions, inrush and ageing, then choose the next standard supply rating. Common ratings are 2.5, 5, 10, 20 and 40 A.
Supplies derate at high ambient temperature, so check the derating curve for your panel. Keep sensors and field loads on a separate protected output from the PLC where possible.
Common questions
How much headroom should a 24 V supply have?
Twenty to thirty per cent is a common starting point. Add more if the panel will be extended, if loads have high inrush, or if the panel runs hot.
Is it better to use one big supply or several smaller ones?
Separate supplies for the PLC and for field loads stop a field short circuit from dropping the controller. Many panels use a main supply with electronic protection on each branch.
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